Pancreatic beta cells within the Islets of Langerhans are required for glucose-stimulated insulin secretion and glucose homeostasis. Dysfunction in beta cell activity or survival results in diabetes mellitus, a disease currently affecting millionsof Americans with numbers expected to greatly increase in the future, thus creating an enormous economic and health burden. A future strategy to improve outcomes for the growing numbers of diabetic patients requires understanding the complex developmental programs that specify and differentiate functional beta cells from progenitors. Exploiting existing knowledge and future understanding of the signaling and transcriptional events central to beta-cell biology will allow u to produce therapeutic replacement beta cells for transplantation. Central to these Aims is the hypothesis that the transcriptional cofactor, Ldb1, is required for gene regulation mediated by LIM-domain transcription factors to produce functional beta cells during development and in adults. My preliminary data defines the expression pattern of Ldb1 in the pancreas as well as the role of Ldb1 in developing endocrine cells. Conditionally deleted mice demonstrated that Ldb1 developmental function partially overlaps with the only well-studied LIM factor in the pancreas, Islet-1 (Isl1). These Aims will further define the Isl1-dependent and -independent genetic events controlled by Ldb1. Aim 1 will utilize unbiased genome-wide microarray and ChIP-Seq approaches to compare the genes and pathways regulated by Ldb1 and Isl1 during pancreas development. Aim 2 will isolate and compare Ldb1- and Isl1-interacting co-regulators from pancreatic endocrine cells using an innovative immunoprecipitation procedure, as I hypothesize that Ldb1 and Isl1 recruit unique subsets of interacting factors in the pancreas. In Aim 3, I will test new Aim 2 candidate binding proteins for their potential roles in Ldb1- or Isl1-mediated transcriptional control utilizing in vivo knockout and cell line knockdown models. Ldb1:Isl1 shared target genes including MafA and Arx or those identified in Aim 1 will be tested for regulation by new interactors. With this K01 Career Developmental Award, I will test my overall hypothesis that Ldb1 controls an array of islet target genes that include Isl1- and (potentially) LMO-mediated complexes in part through interactions with distinct binding coregulators. My current environment is uniquely suited for me to successfully initiate the proposed studies and further training. I will develop the tools and skills to answer many of the questions raised by these Aims and generate interesting data allowing me to transition into funding as an independent investigator.